In automobiles, light trailers, and other light-duty applications, it is common for a wheel bearing assembly to be sealed relatively permanently for all or a good part of the service life of the assembly. Thus, automotive applications, permanently greased bearing assemblies are covered by opaque hubcaps (as opposed to decorative wheel covers which are sometimes incorrectly called hubcaps) which are never removed except for repairs, or for replacement of bearings and/or seals. In most cases, the bearings are permanently lubricated, or are lubricated by a supply of grease coming from another portion of the sealed assembly as a whole.
In the case of large, over-the-road truck tractors and semi-trailers, and other heavy-duty applications, however, the situation is different. Vehicles of these types have extended service lives both in terms of years and in terms of miles traveled. Wheel bearing assemblies are critical components of vehicles such as these. Most commonly, these wheel assemblies include a large, heavy-duty spindle and bearings engaging one or more rotatable parts, including a rotatable hub portion that in turns supports a brake drum and a removable wheel. In driven wheel applications, the bearings usually rest in the axle housing so that the driving axle is not subject to bending loads. In these cases, while the axle housing takes most or all of the load transmitted from the tires and wheels, and these bearing sets also include one or more oil or grease seals to retain the lubricant within the sealed region, and therefore also require hubcaps.
A number of years ago, many manufacturers and fleet operators, with the aid of bearing and seal suppliers, discovered that the life of ball or roller bearing type assemblies could be extended substantially by using oil rather than grease as the lubricant. Thus, the oil, although more fluent (less viscous) than grease, successfully addressed the problems of flushing dirt from the interfaces between metal parts. Modem oils are also formulated in such a way as to suspend entrained dirt and various other residues within the oil. In this way, as in automotive crankcases or other applications, the oil can be periodically drained and replaced with new, fresh oil, thus removing the grit and contaminants. This concept was brought to practical reality by the development of seals capable of handling low viscosity oils opposed to merely being able to impede leakage of semi-solid or gel materials such as greases.
Numerous prior art hubcap designs have been disclosed in U.S. Pat. No. 3,064,982 to Stephens, U.S. Pat. No. 3,316,022 to Isenbarger, U.S. Pat. No. 4,073,540 to Jackowski, U.S. Pat. No. 5,024,488 to Lindhuber et al., and U.S. Pat. No. 5,505,525, to Denton. To the extend effective, these prior art design had disadvantages. For example, of the prior art designs included a vent plug with internal pockets wherein oil tension lead to oil accumulation and the potential for light oil leakage. Another prior art vent plug design taught a venting section formed as “+”, i.e. “plus” shaped vent, having several wedges designed to permit passage of gas and vapor therethrough. Unfortunately, when in a non-vented mode, these wedges of the venting section failed to close properly and left voids therebetween to permit liquid, i.e. oil to escape from the sealed region and allow debris from outside to enter the voids thereby reducing lifespan of the hubcaps.
Therefore, an opportunity exists for an improve design of the vent plug that will prevent debris to enter into the hubcaps thereby increasing lifespan of the hubcaps and allowing oil not to flow away to the outside from the hubcap.
Another opportunity exists for an improve design of the vent plug that will open when pressurized to release the internal pressure at the same time allowing for minimal oil leakage when opened.
Still another opportunity exists for an improve design of the vent plug that is oil-impermeable and provide a positive mechanical barrier against the entrance of contaminants of all kinds.